Positioning correction method for motor-operated injection molding machine

ABSTRACT

A positioning correction method for a motor-operated injection molding machine, intended to eliminate waste of power supply to or overheat of servomotors by locking up in an ideal state the motor-operated injection molding machine, which uses a toggle mechanism or crank for mold clamping and other operations. During the lockup period, a load RA acting on a mold clamping servomotor 7 is detected (S8), and a position command value PM for the servomotor 7 is increased or decreased in the next cycle in a direction such that the value of the load RA is reduced, if the value of the load RA is greater than a preset reference value V0 (S12, S13). As a result, a rotation center Q1 of a crank 2 and two opposite ends Q2 of a link 3 are locked up in a straight line, so that a moment of rotation produced by a mold clamping reaction force is removed from the rotation center Q1 of the crank 2. In this manner, the mold clamping reaction force can be maintained during the lockup period without applying a driving torque to the mold clamping servomotor 7 by energizing the same. Thus, not only the waste of power can be prevented but also the load on the mold clamping servomotor 7 can be reduced, thereby unnecessitating the use of a stopper or other means for maintaining the mold clamping reaction force.

TECHNICAL FIELD

The present invention relates to a positioning method of moving/pressingmeans for moving and pressing a movable member of an injection moldingmachine against a stationary member through drive control by aservomotor.

BACKGROUND ART

Conventionally known are motor-operated injection molding machines whichuse servomotors for driving a mold clamping mechanism, nozzle touchingmechanism, etc.

The motor-operated injection molding machines which utilize a servomotorfor the mold clamping mechanism are classified broadly into twocategories: a straight-hydraulic type, in which a movable platen fittedwith a movable-side mold is linearly moved for mold clamping by drivinga ball-nut-and-screw mechanism by means of the servomotor, and a type inwhich the mold clamping is effected by pushing out the movable platen bymeans of a link mechanism. In general, the link mechanism of the lattertype may be formed of a toggle mechanism or a crank mechanism.

Referring now to the diagrams of FIGS. 4 and 5, the respectiveoperations of mold clamping mechanisms, in which the movable platen ispressed against a stationary platen by means of the link mechanism, willbe described. In FIG. 4, which illustrates the operation of a linkmechanism using a crank, symbol 1a designates the crank, which isrotated around a support joint Q1 by means of a servomotor M. A drivingjoint Q2 of the crank 1a is connected to an action joint Q3, on the sideof a movable platen mp for use as a movable member through a link 1b.When the crank 1a is rotated in the clockwise direction of FIG. 4 bydriving the servomotor M in this arrangement, the movable platen mplinearly moves in the direction of the arrow of FIG. 4 (i.e., toward thestationary platen) to come into contact with the stationary platen,thereby producing a mold clamping force. Thereupon, positioning iseffected by commanding the servomotor M to take a position such that thedriving joint Q2 is located on a line which connects the support jointQ1 and the action joint Q3, after adjusting the position of the supportjoint Q1 or the like so that a set mold clamping force is produced in astate (lockup state) such that the support joint Q1, driving joint Q2,and action joint Q3 are situated substantially on a straight line, asindicated by dotted line in FIG. 4. In the state that the set moldclamping force is obtained with the support joint Q1, driving joint Q2,and action joint Q3 arranged on a straight line, as a result, the crank1a and the link 1b receive a reaction force against the mold clampingforce in the lockup state in which they are stretched to their fulllength and situated on a straight line, so that no rotatory force isapplied to the crank 1a. Thus, no external force acts so as to rotatethe servomotor.

FIG. 5 is a diagram for illustrating the operation of a mold clampingmechanism using a link mechanism of the (double) toggle type. Theprinciple of operation of this mechanism resembles that of the one usingthe crank mechanism of FIG. 4.

More specifically, when a ball screw bs is driven by a servomotor M, atoggle head th, which is integral with a nut threadedly engaged with theball screw bs, linearly moves. As this is done, the movable platen movestoward the stationary platen in a manner such that links 1a and 1b andlinks 1c and 1d gradually shift their respective postures from bentpositions, indicated by full lines in FIG. 5, to stretched positions,indicated by dotted lines. Thereupon, the servomotor is commanded totake a position for a set mold clamping force (position such that asupport joint Q1, driving joint Q2, and action joint Q3 are situated ona straight line), after adjusting the position of the support joint Q1or the like so that the set mold clamping force is produced in a state(lockup state) such that the support joint Q1, driving joint Q2, andaction joint Q3 are situated substantially on a straight line, asindicated by each dotted line in FIG. 5. In the state that the set moldclamping force is obtained in this manner, as in the case of FIG. 4, thelinks 1a and 1b and the links 1c and 1d receive a reaction force againstthe mold clamping force in the lockup state, in which the links in eachset are stretched to their full length and situated on a straight line,so that no rotatory force is applied to the link 1a. Thus, no externalforce acts to rotate the servomotor. Although the toggle shown in FIG. 5is of the double-toggle type, a single-toggle type is based on the sameprinciple, so that its description will be omitted here.

Although the prior-art principle of operation in the case where the moldclamping mechanism is driven by means of the link mechanism using thecrank or toggle has been described, this link mechanism is also utilizedin a nozzle touching mechanism. More specifically, the link mechanismusing the crank or toggle is utilized for moving an injection apparatus,as a movable member, toward a stationary mold and pressing a nozzle ofthe injection apparatus against the stationary platen to produce andmaintain a predetermined touching force. This principle of operation ofthe nozzle touching mechanism is equivalent to that of an arrangement inwhich the movable platen mp of FIG. 4 is replaced with the injectionapparatus (which is moved toward the stationary mold during nozzletouching operation) in the case of the nozzle touching mechanism usingthe crank mechanism, and to that of an arrangement in which the movableplaten mp of FIG. 5 is replaced with the injection apparatus in the caseof the nozzle touching mechanism using the toggle mechanism. Since thesearrangements are based on the same principle as the ones described withreference to FIGS. 4 and 5, individually, further description thereofwill be omitted here.

In performing the mold clamping or nozzle touching operation through thelink mechanism using the crank or toggle, as described above, anassigned position for the servomotor is judged such that the one link(or crank) and the other are in the lockup state in which they arestretched substantially to their full length. When the delivery of theposition command to the servomotor is finished, the link mechanism islocked up by being positioned in this manner, so that the force from themold clamping mechanism acting around the axis of the servomotor can bethoroughly removed. Consequently, no load acts on the servomotor, sothat only a very small driving current is needed to keep the servomotorin its rotational position. Thus, a predetermined mold clamping state ornozzle touching state can be maintained with a supply of fine current.

According to the motor-operated injection molding machine describedabove, however, various problems arise in the positioning control forthe servomotor due to secular changes such as changes in dimensions ofvarious parts of mechanisms or changes in friction coefficient, whichare attributable to fit abrasion, deterioration of relative dimensionalaccuracy attributable to local temperature changes, etc.

In case of fit abrasion at part of a drive transmission section of themold clamping mechanism which uses the toggle mechanism or crank formold clamping, for example, the toggle mechanism or crank may sometimesfail to reach a predetermined position (position for the lockup state),or may possibly move beyond the predetermined position, even when theservomotor is controlled so as to be driven to a preset commandposition. In such a case, a predetermined mold clamping force or nozzletouching force cannot be obtained or maintained, so that the moldclamping mechanism or nozzle touching mechanism cannot establish andmaintain a precise lockup state. Thus, in this state, a reaction forcefrom the mold clamping mechanism or a reaction force based on the nozzletouching force acts around the axis of the servomotor, so that a drivingcurrent corresponding to the reaction force must be suppliedcontinuously during the mold clamping period or nozzle touching period,in order to control the servomotor so that its present position ismaintained. In some cases, therefore, the servomotor may overheat.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a positioningcorrection method for a motor-operated injection molding machine,capable of eliminating the drawbacks of the prior art, and preciselycontrolling the drive of various parts of mechanisms of themotor-operated injection molding machine, regardless of various secularchanges and dimensional changes.

In order to achieve the above object, according to the presentinvention, there is provided a motor-operated injection molding machine,which is designed so that a movable member is moved toward a stationarymember to be brought into contact therewith by the drive control of aservomotor through a link mechanism, and a support joint, a drivingjoint, and an action joint of the link mechanism are arranged on astraight line for positioning such that a predetermined force ofpressure goes on acting, and in which a load acting on the servomotorwhen the movement of the servomotor to an assigned position is completedis detected, and a position command for the movement of the servomotorto the assigned position for the next cycle is modified on the basis ofthe relationship between the value of the detected load and the value ofa preset reference load so that the movable means goes on applying thepredetermined force of pressure to the stationary member in the nextcycle under a stabler mechanical condition than in the preceding cycle.

Preferably, the movable member is a movable platen fitted with a movablemold member or an injection apparatus, and a set mold clamping force ora nozzle touching force is produced when the support joint, drivingjoint and action joint of the link mechanism are arranged on a straightline. Further preferably, the link mechanism is of either a crank ortoggle type, and a command position for the servomotor is a positionsuch that the support joint, driving joint and action joint of a crankor toggle of the link mechanism are arranged on a straight line.

Further preferably, the assigned position for the next cycle is obtainedby adding the product of the value of the load acting on the servomotorand a preset coefficient to the value for the assigned position for thepreceding cycle, if the absolute value of the load acting on theservomotor when the movement of the servomotor to the assigned positionis completed is greater than the value of the preset reference load.Alternatively, the assigned position for the next cycle is obtained byadding or subtracting a preset fixed value to or from the value for theassigned position for the preceding cycle.

Further preferably, the value of the load on the servomotor is theaverage of the respective values of loads sampled at a plurality ofpoints of time in the same cycle.

According to the present invention, as described above, the load on theservomotor is detected as the movable member is pressed against thestationary member by locking up the crank or toggle by using theservomotor as a drive source, and the value of the position command forthe servomotor is automatically corrected so that the detected loadcomes within a set range. Thus, even in the case of secular changes,such as changes in dimensions of various parts mechanisms or changes infriction coefficient, which are attributable to fit abrasion,deterioration of relative dimensional accuracy due to local temperaturechanges, etc., the crank or toggle can be precisely moved to amechanically stable point, so that the lockup can be kept in an idealstate such that no external force acts on the servomotor. Thus, waste ofpower supply to the servomotor and overheat of the servomotor can beprevented during the lockup period, and the trouble of providing amechanical stopper for maintaining the lockup can be eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the principal part of a crank-typemotor-operated injection molding machine according to one embodiment forcarrying out a method of the present invention;

FIG. 2 is a flow chart showing an outline of a positioning correctionprocessing executed in task processing for each predetermined period bymeans of a PMCCPU provided for the injection molding machine of theembodiment;

FIG. 3 shows diagram illustrating states of a crank machanism in theinjection molding machine of the embodiment, in which FIG. 3(a)illustrates a mold opening completion state, FIG. 3(b) illustrates anideal lockup state, FIG. 3(c) illustrates a state such that a crank isnot fully rotated, and FIG. 3(d) illustrates a state such that the crankis rotated beyond the lockup state;

FIG. 4 is an operation principle diagram for illustrating an operationin which a movable platen is moved in a straight-line direction by meansof a link mechanism using a crank, driven by means of a servomotor; and

FIG. 5 is an operation principle diagram for illustrating an operationin which a movable platen is moved in a straight-line direction by meansof a link mechanism using a toggle, driven by means of a servomotor.

BEST MODE OF CARRYING OUT THE INVENTION

FIG. 1 is a block diagram showing the principal part of a motor-operatedinjection molding machine according to one embodiment for carrying out amethod of the present invention. Illustrated in this diagram is only thesurroundings of a mold clamping mechanism in the crank-type injectionmolding machine. In FIG. 1, tie bars 5 are stretched between a rearplaten 1 and a front platen, and a movable platen 6 is slidably mountedon the tie bars 5. Further, a crank 2 of a crank mechanism 4, whichconstitutes the mold clamping mechanism, is rotatably mounted on therear platen 1. One end of a link 3 is pivotally mounted on a bentportion of the crank 2, while the other end of the link 3 is pivotallymounted on the reverse side (side remoter from molds) of the movableplaten 6. The crank 2 is rotated by means of a servomotor 7 for moldclamping. This servomotor 7 is provided with a pulse coder 8 fordetecting its rotational position and speed.

On the other hand, the motor-operated injection molding machine iscontrolled by means of a numerical control device 100. The numericalcontrol device 100 comprises a microprocessor (hereinafter referred toas NCCPU) 108 for NC and a microprocessor (hereinafter referred to asPMCCPU) 110 for programmable machine controller. The PMCCPU 110 isconnected to a ROM 113 stored with sequence programs for controlling thesequence operation of the injection molding machine and the like, a RAM106 used for temporary storage of data and the like, and a RAM 116loaded with a torque command voltage mentioned later. The NCCPU 108 isconnected to a ROM 111 stored with management programs for generallycontrolling the injection molding machine and the like, a RAM 102 usedfor temporary storage of data and the like, and a servo interface 107.

The respective buses of the NCCPU 108 and the PMCCPU 110, along with thebus of a common RAM 103 and the respective buses of an input circuit 104and an output circuit 105, are connected to a bus arbiter controller(hereinafter referred to as BAC) 109. The buses used are controlled bymeans of the BAC 109. The common RAM 103, which is a nonvolatile commonRAM composed of a bubble memory or CMOS memory, includes a memorysection for storing NC programs for controlling individual operations ofthe injection molding machine and the like, a set memory section forstoring various set values, parameters, and macro variables associatedwith molding conditions and the like, etc.

The servo interface 107 is connected to servo circuits 101 which areused to control servomotors for individual axes for injection, moldclamping, screw rotation, ejector operation, etc. The servo system shownin FIG. 1 includes only the mold clamping servomotor 7 and the servocircuit 101 therefor. An output from the pulse coder 8 is applied to theinput of the servo circuit 101.

Further, a manual data input device 114 with CRT (hereinafter referredto as CRT/MDI) display unit is connected to the BAC 109 through anoperator panel controller 112. Various set pictures and operation menuscan be displayed on a CRT display screen of the CRT/MDI 114. Byoperating various operation keys (soft keys, ten-keys, etc.), moreover,various set data and set pictures can be inputted and selected,respectively.

In the arrangement described above, the numerical control device 100 isdesigned so that the NCCPU 108 distributes pulses to the servo circuitsfor the individual axes of the injection molding machine through theinterface 107, thereby controlling the injection molding machine, as thePMCCPU 110 performs sequence control operation in accordance with the NCprograms stored in the common RAM 103, the parameters for the variousmolding conditions stored in the set memory section of the common RAM103, and the sequence programs stored in the ROM 113.

The servo circuit 101 substracts pulses, which are from the pulse coder8, from the distributed pulses received through the servo interface 107,and causes an error register to output an error quantity for the presentposition compared with a command position. Then, the output of the errorregister is D/A-converted and delivered as a speed command; this speedcommand is compared with the present speed obtained by F/V-convertingthe output of the pulse coder 8, and a current command, that is, torquecommand, for the mold clamping servomotor 7 is outputted. Also, thecurrent presently flowing through the mold clamping servomotor 7 iscompared with the current command delivered from a speed controller, andthe current for the servomotor 7 is controlled to regulate the outputtorque in accordance with the result of the comparison.

Further, a torque command voltage detected in the servo circuit 101 isloaded, as a driving torque for the mold clamping servomotor 7, into theRAM 116 through an A/D converter 115 with every predetermined period.The PMCCPU 110 detects the data written in this RAM. Since theoperations for position, speed, and current loop processing associatedwith the servomotor control are generally known, further detaileddescription of these operations will be omitted.

FIG. 3 shows diagrams for illustrating the respective behaviors of thecrank 2 and the link 3 shown in FIG. 1 during mold clamping operation.While the direction of the axis of rotation of the crank 2 is verticalin FIG. 1, it is perpendicular to the drawing plane of FIG. 3. In FIG.3, Q1 denotes a support joint as the axis of rotation of the crank, andthe one and the other ends of the link 3 are connected to a drivingjoint Q2 and an action joint Q3, respectively. The dashed line of FIG.3(a) is a straight line which, passing through the support joint Q1,represents a path of transfer of the support joint Q1.

FIG. 3(a) is a diagram showing a state of the crank mechanism 4established on completion of mold opening, while FIG. 3(b) is a diagramshowing a state in which the crank mechanism is locked up normally. Inthis lockup state, the support joint Q1, driving joint Q2, and actionjoint Q3 are situated on a straight line such that the link 3 isstretched (or locked up) to its full length, so that a reaction forceproduced by mold clamping does not act as a moment of rotation aroundthe support joint Q1. Accordingly, no external load will be applied tothe mold clamping servomotor 7.

On receiving a position command PM for a movement corresponding to arotational angle θ of the crank 2, the mold clamping servomotor 7rotates the crank 2 to move the operating position of each part of thecrank mechanism 4 from the mold opening completion state shown in FIG.3(a) to the lockup state shown in FIG. 3(b). The rotational angle θ ofthe crank 2 is set variously according to the mold opening distance,which depends on the mold configuration. Also, the position of the rearplaten 1 itself and therefore the position of the center Q1 of rotationof the crank 2 vary depending on the mold thickness.

If the crank mechanism 4 is in the ideal lockup state, as shown in FIG.3(b) when the rotation of the crank 2 based on the position command PMis finished, a reaction force against a mold clamping force, which isproduced by the lockup of the crank mechanism 4 and extension of the tiebars 5, acts on the crank 2 in the direction of the dashed line of FIG.3(a). After the lockup state is established, therefore, no externalforce acts on the mold clamping servomotor 7.

If a speed reduction mechanism between the mold clamping servomotor 7and the crank 2 has suffered fit abrasion or the like which isattributable to secular changes, however, this will cause backlash orthe like in the speed reduction mechanism, so that the rotation of thecrank 2 will be insufficient. In some cases, it can happen therefore,that the crank mechanism 4 fails to reach an ideal lockup position, asshown in FIG. 3(c), even when the mold clamping servomotor 7 itself isrotated to a commanded position corresponding to the position commandPM. In other cases, however, the crank mechanism 4 may possibly gobeyond the ideal lockup position, as shown in FIG. 3(d), depending onvarious other changes in situation, despite the normal operation of themold clamping servomotor 7 in response to the position command PM.

Even when the complete lockup state is not established, as shown inFIGS. 3(c) and 3(d), however, a mold of the movable platen 6 and a moldof a stationary platen are in contact with each other to produce themold clamping force, so that the moment of rotation produced by the moldclamping force acts in the clockwise direction around the support jointQ1 through the link 3 in the state of FIG. 3(c), and in thecounterclockwise direction in the state of FIG. 3(d). Accordingly, themold clamping servomotor 7 is urged to maintain a set mold clampingposition, resisting the rotatory force thereof. That is, if the positionof the servomotor 7 is deviated from the set mold clamping position dueto the reaction force against the mold clamping force, thereby producinga positional deviation, the servomotor 7 is supplied with such anelectric current as to reduce the positional deviation to zero. Asdescribed above, if the support joint Q1, driving joint Q2, and actionjoint Q3 are situated on a straight line in the ideal lockup state, asshown in FIG. 3(b), the rotatory force acting on the servomotor 7, urgedby the reaction force against the mold clamping force, is substantiallyzero. If the lockup is not complete, that is, if the support joint Q1,driving joint Q2, and action joint are not situated on a straight line,as shown in FIGS. 3(c) and 3(d), however, the force acting on theservomotor 7 increases in proportion to the deviation from the straightline, so that the torque command and driving current for the servomotor7 increase correspondingly.

Referring now to the flow chart of FIG. 2, an outline of a positioningcorrection processing, executed in task processing for eachpredetermined period by the PMCCPU 110, will be described. The period ofthis positioning correction processing is set to be shorter enough thanthe time during which the mold clamping mechanism maintains the lockupstate, and this processing is repeatedly executed a plurality of timesduring one lockup cycle. In the flow chart of FIG. 2, the processings ofSteps S2 to S9 are processing for detecting the average of loads actingon the mold clamping servomotor 7 during one lockup period, while theprocessing of Steps S11 to S14 are processings for modifying the valueof the position command for mold clamping.

Before executing this positioning correction processing, a samplingexecution frequency N and reference load V0 are set by CRT/MDI 114, andstored beforehand.

After starting the positioning correction processing, the PMCCPU 110first judges whether or not a flag in the common RAM 103, which isindicative of the lockup state of the crank mechanism as an object, thatis, a lockup period flag Fl, is set by means of the NCCPU 108, and alsowhether a sampling completion flag Fs is set or not, therebydiscriminating between a state that the mold clamping mechanism ispresently locked up and a state that sampling is not completed (StepS1). If the lockup period flag Fl is not set, or if the samplingcompletion flag Fs is set, then the above-described state will not bediscriminated, so that the result of judgement in Step S1 will benegative (N), and thus the PMCCPU 110 proceeds to a discriminationprocessing of Step S10 without executing the processings of Steps S2 toS9. Whereas, the processings of Steps S2 to S9 will be executed duringthe period of the aforesaid state in which the mold clamping mechanismis locked up (Fl=1), and the sampling for the load detection during thelockup state is not completed (Fs=0), since the result of judgement inStep S1 will become positive (Y).

If the result of judgement in Step S1 is negative, the PMCCPU 110proceeds to Step S10, whereupon it further judges whether the lockupperiod flag Fl is set or not, and whether the sampling completion flagFs is set or not. If the lockup period flag Fl is reset (F1=0), and ifthe sampling for the molding cycle concerned is not finished, the resultof decision in Step S10 will be negative (N). Thereupon, the PMCCPU 110finishes the processing for the present period without executing theprocessings of Steps S11 to S14. The processings of Steps S11 to S14 areexecuted only during an initial processing period at the start of themold opening, as mentioned later.

Meanwhile, if a mold clamping start command is outputted in accordancewith the sequence programs of the PMCCPU 110 when the crank mechanism 4is in the mold opening completion position, as shown in FIG. 3(a), theNCCPU 108 distributes in pulses the parameter PM for mold clamping setin the common RAM 103, that is, the position command PM for the movementcorresponding to the rotational angle θ of the crank 2, thereby startingthe drive of the mold clamping servomotor 7 through the medium of theservo interface 107 and the servo circuit 101 to lock up the crankmechanism 4. Normally, when the position of the mold clamping servomotor7 gets into a target-in-position range so that the lockup state isestablished after the pulse distribution is finished, the positionaldeviation associated with the mold clamping servomotor 7 is reduced tozero or a very small value approximate to zero. Further, the NCCPU 108detects that the position of the mold clamping servomotor 7 has enteredthe target-in-position range, and sets the lockup period flag Fl of thecommon RAM 103.

Thereupon, when the PMCCPU 110 detects in the discrimination processingof Step S1 that the lockup period flag Fl is set, and that thesampling-completion flag Fs is not set, (this flag Fs is not set at thestart), it then judges whether a sampling start flag Fi is set or not(Step S2). Since this flag is not set (Fi=0) at the present stage or inthe initial period of the lockup state, the PMCCPU 110 executes a firstsampling cycle for detecting the average value of loads acting on themold clamping servomotor 7 during the present lockup period. Morespecifically, the PMCCPU 110 reads an up-to-date present value Vn of thedriving torque of the mold clamping servomotor 7 from the RAM 116,stores it in a average load storage register RA (Step S5), sets a value(N-1) to be obtained by subtracting 1 from the previously set samplingexecution frequency N in a sampling counter C, and further sets thesampling start flag Fi (Step S6).

Subsequently, the PMCCPU 110 judges whether the value in the samplingcounter C is 0 or not, that is, whether the sampling cyclescorresponding to the previously set frequency N are completed or not(Step S7). Since the present stage corresponds to a first period ofprocessing in which the value in the counter is not 0 (C≠0), the PMCCPU110 proceeds to Step S10 without executing the processings of Steps S8and S9. Then, the PMCCPU 110 judges whether the lockup period flag Fl isreset or not, and whether the sampling-completion flag Fs is set or not,in the same manner as aforesaid, thereby discriminating the resettingand setting, respectively, of the lockup period flag Fl and thesampling-completion flag Fs (Step S10). At the present stage, the lockupperiod flag Fl is not reset (Fl≠0), and the sampling completion flag Fsis not set (Fs≠1) either, so that the PMCCPU 110 finishes thepositioning correction processing for this period without executing theprocessings of Steps S11 to S14, and returns to the same sequenceprocessing as the conventional one.

In the positioning correction processing for the next period, the PMCCPU110 first detects in this stage that the lockup period flag Fl is setand that the sampling completion flag Fs is not set (Fl=1 AND Fs=0), sothat the result of decision in Step S1 is positive (Y), and the PMCCPU110 proceeds to Step S2. Thereupon, the PMCCPU 110 detects that thesampling start flag Fi is set (since the present period is not the firstone), so that it reads the up-to-date present value Vn of the drivingtorque of the mold clamping servomotor 7 from the RAM 116, adds thisdata to the value in the average load storage register RA, and executesa second sampling cycle (Step S3). Further, the PMCCPU 110 decrementsthe value in the sampling counter C by 1 (Step S4).

Then, the PMCCPU 110 executes the discrimination processings of Steps S7and S10 in which the result of decision is negative (N), in the samemanner as aforesaid, and finishes the positioning correction processingfor this period.

In individual subsequent periods, the PMCCPU 110 repeatedly executes thediscrimination processings of Steps S1 and S2, sampling processing ofStep S3, processing of Step S4, and discrimination processings of StepsS7 and S10, in the same manner as aforesaid.

When the PMCCPU 110 detects that the sampling processing of Step S3,including the processing of Step S5, is executed for the set frequency(N number of cycles) so that the value in the sampling counter C becomes0 (Step S7), the cumulative value stored in the average load storageregister RA is divided by the sampling frequency N to calculate theaverage value of the driving torques of the mold clamping servomotor 7,and the resulting value is stored in the average load storage registerRA (Steps S8). The sampling start flag Fi is reset, while the samplingcompletion flag Fs is set (Step S9).

Then, the PMCCPU 110 discriminates between a state that the lockupperiod flag Fl is reset and a state that the sampling completion flag Fsis set, respectively to judge whether the lockup state and thecalculation of the average value of the driving torques are finished ornot (Step S10). Since the flag Fl is set (F≠0) during the lockup state,the result of judgement in Step S10 is negative (N). During the timeinterval in which an injection/dwell stage and cooling stage arefinished, and the crank mechanism 4 is released from the lockup state(Fl=0; and Fs=1 is previously established), and thereafter, the PMCCPU110 repeatedly executes only the discrimination processings of Steps S1and S10.

When the crank mechanism 4 is released from the lockup state so that thelockup period flag Fl is reset (i.e., Fl=0 AND Fs=1) after the end ofthe injection/dwell stage and cooling stage, the result of judgement inStep S10 becomes positive (Y), whereupon the PMCCPU 110 compares theaverage value (absolute value) of the driving torques of the moldclamping servomotor 7, stored in the average load storage register RA,and the previously set reference load V0 (>0) (Step S11). |RA|≦V0 is thebasis for judging whether the average value of the driving torquesstored in the register RA is between -V0 and +V0 (-V0≦RA≦+V0) or not. Ifthe crank mechanism 4 is locked in the ideal state, as shown in FIG.3(b), no external force acts on the mold clamping servomotor 7 in thelockup state, so that the cumulative value and average value RA of thedriving torques stored in the register RA are theoretically zero, thatis, |RA|≦V0 holds. In this case, the value of the parameter PM storedwith the position command for mold clamping requires no correction.

In the case of FIG. 3(c) where the crank mechanism 4 cannot come closeto the ideal lockup position even when the mold clamping servomotor 7 isrotated to the commanded position PM, however, the relationship betweenthe average driving torque value and the reference load can be given byRA>V0. On the other hand, in the case of FIG. 3(d) where the ideallockup position is overshot by the crank mechanism 4 when the moldclamping servomotor 7 is rotated to the command position PM, RA<-V0 isobtained. In these cases, the servo circuit 101 must go on deliveringforward and reverse torque command currents throughout the lockupperiod, in order to keep the mold clamping servomotor 7 in the commandposition. The greater the value of the load |RA| on the mold clampingservomotor 7, the higher the power consumption is.

If the PMCCPU 110 concludes in the processing of Step S11 that there isa relation |RA|>V0, therefore, the value in the average load storageregister RA is multiplied by a proportionality factor k to calculate acorrection value PS for the position command (Step S12), and thecorrection value PS is added to the value of the parameter PM storedwith the position command for mold clamping, thereby setting a newposition command value PM again (Step S13). Since this correctedposition command value is delivered to the mold clamping servomotor 7 inthe next molding cycle, the lockup position of the crank mechanism 4approaches the ideal state shown in FIG. 3(b). That is, if the statebefore the correction is the one shown in FIG. 3(c), the value stored inthe average load storage register RA exceeds the reference load V0(RA>V0), so that the value PS (=k·RA) is positive, and the new positioncommand value PM increases correspondingly. In the next mold clampingcycle, therefore, the movement of the crank mechanism 4 becomes largerthan that of the preceding claming cycle so that the position of themechanism 4 approaches the ideal lockup position, as shown in FIG. 3(b).On the other hand, in the case of the lockup position shown FIG. 3(d),there is the relation RA<-V0, so that the value PS (=k·RA) is negative,and the new position command value PM is reduced correspondingly. In thenext mold clamping cycle, therefore, the movement of the crank mechanism4 is reduced, so that its moved position also approaches the ideallockup position.

In the present embodiment, the correction value PS of the positioncommand is calculated by multiplying the value in the average loadstorage register RA by the proportionality factor k, so that thecorrection value for the position command value PM can be obtained by asingle processing for correction. In executing this positioningcorrection processing, which is executed every time the link mechanism 4is locked up, the processing of Step S12 may be omitted. Since theposition command value PM can be either incremented or decrement by apredetermined value at a time in the processing of Step S13, in whichthe position of the crank mechanism 4 can be modified by a fixeddistance for each mold clamping cycle so that the crank mechanism 4 canbe is driven to the ideal position (i.e., position where |RA|≦V0 holds).Actually, the average value RA of the driving torques non-linearlyvaries depending on the angular deviation of the crank 2 from the ideallockup position, so that a coefficient which can replace theproportionality factor may be calculated on the basis of this non-linearfunction. However, since the positioning correction processing is aprocessing executed every time the link mechanism 4 is locked up, asmentioned before, the moved position of the crank mechanism 4 cangradually be modified for each mold clamping cycle so that the mechanism4 is driven to the ideal position. Thus, the object can be fullyachieved by using the simple proportionality factor.

After thus correcting the parameter value PM stored with the positioncommand value; the PMCCPU 110 initializes the sampling completion flagFs (Step S14), and then finishes the positioning correction processingfor this period.

During the time interval from the start of mold opening following theinjection/dwell stage and cooling stage to the output of the next moldclampling start signal, lookup period flag Fl is reset, and the samplingcompletion flag Fs is kept unset. Accordingly, the results of decisionin Steps S1 and S10 are both negative (N), so that only thediscrimination processings of Steps S1 and S10 are repeatedly executed,and the positioning correction processing will not be executed. When thelockup state of the crank mechanism 4 is detected, thereafter, thepositioning correction processing will be repeatedly executed for eachof the present and subsequent periods, in the same manner as aforesaid,whereby the lockup position of the crank mechanism 4 will be correctedaccording to the preceding cycle of load detection.

Thus, according to the present embodiment, even when the ideal lockupstate cannot be obtained due to fit abrasion, which is attributable tosecular changes, or localized temperature variation of the members, thesupport joint Q1, driving joint Q2, and action joint Q3 of the crank 2of the crank mechanism 4 can be situated on a straight line to removethe mold clamping reaction force from the mold clamping servomotor 7 bygradually correcting the rotational position of the crank 2.Consequently, the mold clamping reaction force need not be maintainedduring the lockup period by means of the driving force of the moldclamping servomotor 7, so that not only wasteful power consumption canbe prevented but also mechanical stopper means for maintaining the moldclamping reaction force need not be provided.

Although the above-described embodiment is an example of application ofthe present invention to the crank-type mold clamping mechanism, thepresent invention may be also applied to a toggle-type mold clampingmechanism driven by a servomotor. Like the crank type mentioned before,the toggle-type mold cramping mechanism is designed so that a moldclamping force is produced when a toggle is in a lockup state, that is,a state in which it is stretched to its full length. In this (ideal)state, the mold clamping force will never act as a force to bend thetoggle, so that the drive of the servomotor for driving the togglemechanism will be stopped.

If the toggle fails to be stretched to its full length for some reasonwhen the servomotor is driven to its mold clamping command position,however, the toggle mechanism will not be able to maintain the moldclamping force under its mechanically stable condition, so that the moldclamping force acts as a force to fold the toggle in the aforesaidstate. In order to overcome this mold clamping force and maintain afixed mold clamping force, therefore, an electric current is supplied tothe servomotor.

Thus, according to the present invention, the mold clamping mechanismusing the toggle, like the case of the crank type according to theforegoing embodiment, is arranged so that the load on the servomotorduring the mold clamping period is detected by a driving torque (torquecommand value), and the mold clamping command position is corrected sothat the load to be detected is within a set range, whereby the togglemechanism can maintain the mold clamping force under the mechanicallystable condition.

The above-described embodiments are characterized by that the moldclamping command position of the servomotor for controlling the moldclamping mechanism is appropriately modified so that the mold clampingmechanism, using the crank mechanism or toggle mechanism, can producethe mold clamping force at its mechanically stable point. Suchmodification of the command position may be applied to a nozzle touchingmechanism as well as to the mold clamping mechanism. That is, in movingthe whole injection apparatus to a resin inlet port of the mold by meansof moving/pressing means, which corresponds to the crank mechanism ortoggle mechanism, so that the apparatus is brought into contact with theport, in order to inject a material into the clamped mold, (thetheoretical command value of) a position command to be delivered to aservomotor for controlling the drive of the moving/pressing means isappropriately modified into a value such that the moving means canactually maintain a nozzle touching state under the mechanically stablecondition. Thus, according to this arrangement when the whole injectionapparatus is in the nozzle touching state, the servomotor need not besupplied with any special current for maintaining this state. Morespecifically, as in the case of the foregoing embodiments, the load onthe servomotor during the touching period (lockup period) is detected,and the position to be judged by the servomotor during the touchingperiod is corrected so that this load is within a set range.

Although the analog servo circuit is taken as an example of a circuitfor controlling the drive of the servomotor according to the foregoingembodiments, a digital servo circuit, in which a microprocessor executesprocessings equally to the analog servo circuit, may alternatively beused. In this case, a torque command for the servomotor is handled as adigital value as it is calculated in the digital servo circuit andoutputted, so that the calculated torque command can be read by thePMCCPU 110 through the servo interface 107, NCCPU 108, and common RAM103.

Moreover, according to the foregoing embodiments, the load on theservomotor is detected by the torque command (current command) deliveredto the servomotor. Alternatively, however, the load on the servomotormay be detected by directly detecting the current flowing through theservomotor.

We claim:
 1. A positioning correction method for a motor-operatedinjection molding machine which is designed so that a movable member ismoved toward a stationary member so as to be brought into contacttherewith by the drive control of a servomotor through a link mechanism,and a support joint, a driving joint and an action joint of said linkmechanism are arranged on a straight line for positioning such that apredetermined force of pressure goes on acting, comprising stepsof:detecting a load acting on said servomotor when the movement of saidservomotor to an assigned position is completed; and modifying aposition command for the movement of said servomotor to the assignedposition for the next cycle on the basis of the relationship between thevalue of said detected load and the value of a preset reference load sothat said movable means goes on applying the predetermined force ofpressure to said stationary member in the next cycle under a stablermechanical condition than in the preceding cycle.
 2. A positioningcorrection method for a motor-operated injection molding machineaccording to claim 1, wherein said movable member is a movable platenfitted with a movable mold member, and a set mold clamping force isproduced when the support joint, driving joint and action joint of saidlink mechanism have become arranged on a straight line.
 3. A positioningcorrection method for a motor-operated injection molding machineaccording to claim 2, wherein said link mechanism is of either a crankor toggle type, and a command position for the servomotor is a positionsuch that the support joint, driving joint and action joint of a crankor toggle of said link mechanism are arranged on a straight line.
 4. Apositioning correction method for a motor-operated injection moldingmachine according to claim 1, wherein said movable member is aninjection apparatus, and a nozzle touching force is produced when thesupport joint, driving joint and action joint of said link mechanismhave become arranged on a straight line.
 5. A positioning correctionmethod for a motor-operated injection molding machine according to claim4, wherein said link mechanism uses either a crank or toggle, and acommand position for said servomotor is a position such that the supportjoint, driving joint and action joint of the crank or toggle of saidlink mechanism are arranged on a straight line.
 6. A positioningcorrection method for a motor-operated injection molding machineaccording to any one of claim 5, wherein the value of the load on saidservomotor is the average of the respective values of loads sampled at aplurality of points of time in the same molding cycle.
 7. A positioningcorrection method for a motor-operated injection molding machineaccording to any one of claim 5, wherein the assigned position for thenext cycle is obtained by adding the product of the value of the loadacting on said servomotor and a preset coefficient to the value for theassigned position for the preceding cycle if the absolute value of theload acting on said servomotor when the movement of the servomotor tothe assigned position is completed is greater than the value of thepreset reference load.
 8. A positioning correction method for amotor-operated injection molding machine according to claim 7, whereinthe value of the load on said servomotor is the average of therespective values of loads sampled at a plurality of points of time inthe same molding cycle.
 9. A positioning correction method for amotor-operated injection molding machine according to any one of claim5, wherein the assigned position for the next cycle is obtained byadding or subtracting a preset fixed value to or from the value for theassigned position for the preceding cycle if the absolute value of theload acting on said servomotor when the movement of the servomotor tothe assigned position is completed is greater than the value of thepreset reference load.
 10. A positioning correction method for amotor-operated injection molding machine according to claim 9, whereinthe value of the load on said servomotor is the average of therespective values of loads sampled at a plurality of points of time inthe same molding cycle.